The accurate AC measurement of electrical power at various points of a power grid is becoming more and more important and, at the same time, is getting more and more difficult. The old power distribution model of a few, large power generating stations and a multitude of relatively linear loads is being replaced by a newer model containing a multitude of smaller, and to some degree unpredictable power sources, as well as a multitude of not always linear and often smart (essentially also unpredictable) loads. This change deteriorates power quality and makes AC measurements, grid management and troubleshooting more difficult. Also, the increasing cost of electrical power makes precise calculations of delivered energy and monitoring of power quality important.
There are three main categories of AC power measurement systems: The highest level of accuracy, used typically by the Standard and Calibration Laboratories, developed to reference measurement to the National Standards. These are typically unique installations, not covered by specific regulatory requirements. The next category is high precision AC power measurement systems. In the important case of AC power measurement instruments, usually referred to as Power Analyzers, these would be units meeting the requirements of standards, such as for example International Standard IEC 61000-4-30 “Electromagnetic Compatibility: Part 4-30 Testing and Measurement Techniques—Power Quality Measurement Methods” which relates to Class A measurement methods. These are used where precise measurements are necessary, for example for contractual applications and disputes, verifying compliance with standards, etc. Two different Class A instruments, when measuring the same quantities, should produce matching results within the specified uncertainty for that parameter. The third main category of the AC power measurement system is general use instruments. Generally it is recommended that this group reflect measurement methods and intervals of Class A instruments, with lower precision and processing requirements. It is then classified as Class S. Other instruments including legacy installations, whose operation doesn't reflect methods of Class A, but still meet key accuracy requirements, are summarily called Class B. Irrespective of the class of the AC power measurements they require determination of the voltage, current and timing of the single or multiple phases of the power system in order to perform the measurements.
The whole measurement chain of electrical quantity for power analysis consists of measurement transducer, measurement unit and evaluation unit (as is defined in the ICE 61000-4-30 standard). The measurement transducer converts the input quantity to a level and a kind suitable for the measurement unit and typically has some other functionality, for example signal isolation or overload protection. For example, the measurement transducer may reduce a power line voltage of hundreds of kilovolts to tens of volts. The measurement unit then converts the input quantity, typically to a digital form, suitable for evaluation. Then the evaluation unit, which is typically some form of a computing device, receives and combines data streams from different input channels including for example the output of the measurement unit and a reference unit, and does the required calculations to produce results. Test results can be: recorded, aggregated, automatically evaluated in the real time, displayed on the instrument screen, used to generate alarms, placed in system logs, and send out for external evaluation and storage, etc.
AC power measurement requires information on the voltage, current and the relative position between the two. For multi-phase power systems additional information on the relationship between the multiple phases is also required. Some systems also capture information on the current and voltage of the return line. As a rule, the input quantities are accessible only via measurement transducers. Typically the precision of the transducer and the measurement unit determine the overall precision of the system.
A precise voltage transducer typically has the form of a resistor divider, often with reactive frequency compensation. In some applications a voltage transformer, or an inductive divider (autotransformer), may also be used. Other forms of transducer may rely on a capacitor divider, or electrostatic effects.
In contrast, a precise current transducer has typically the form of a multiple stage transformer, or a shunt resistor. Other kinds of transducer may use, for example, a single core current transformer, a Rogowski coil, a Hall Effect sensor, or a fiber optic sensor. Current clamps, a popular physical form of current sensors, usually employ a single stage current transformer, a Hall effect transducer, or a Rogowski coil.
High precision AC measurements require precise analog to digital conversion and precise timing information. To increase precision and measurement repeatability, the typical implementation in the prior art requires that the sampling frequency is synchronized to the fundamental frequency of the incoming signal and the number of samples per period is constant. Synchronization to the fundamental frequency allows for accurate, repeatable measurements of magnitude, frequency and phase of the tested signal. This method requires that the signal under test is stationary. Testing non-stationary waveforms, e.g. slowly varying, noisy, or jittery for example, typically results in increase of measurement uncertainty.
Data incoming from the input channels is typically passed through an extensive range of signal processing blocks. Basic parameters of the input signals being calculated by these and limits on the uncertainty of the determined results depend primarily on the precision of the measurement transducers, measurement units (A/D conversion generally dominating), the timing reference, and analysis algorithms employed.
Power and power quality measurements are important part of the AC measurements. Power quality is characterized by one or more parameters, some of the wide range of parameters being defined for example in the IEC 61000 “Testing and Measurement Techniques—Power Quality Measurement Methods”, IEEE 1159 “IEEE Recommended Practice for Monitoring Electric Power Quality”, and EN50160 “Voltage Characteristics in Public Distribution Systems” specifications and standards. Computer based evaluation units, supplemented with a Coordinated Universal Time (UTC) block can provide tools to analyze power according to any of these standards.
Accordingly it would be beneficial to provide power utilities, independent electricity producers, electrical engineers and technicians, and others requiring accurate measurements of power systems with a field deployable power system measurement devices providing Class A type performance but in rugged devices of reduced cost and complexity. It would be further beneficial if the same principles allowed such devices according to some embodiments of the device to achieve performance approaching that of reference measurements operating in laboratory conditions.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.